In mining technology one usually distinguishes between two main types of mining, viz. open cut mining and underground mining. This invention relates to the latter category. Within underground mining technology three different main methods can be distinguished.
(1) Mining with and by remaining pillars for timbering and maintaining hollow spaces obtained by mining. This method has a relatively good selectivity, but causes mineral loss, at least temporarily unless the pillars are recovered in the final phase (hereby the method transforms to method 3 below).
(2) Caving by permitting layers or blocks and overlying rock to collapse down into hollow spaces resulting from extraction in order thereby to fill the spaces and create a protective layer against collapsing rock parts and blocks. This method has poor or almost no selectivity. It causes, substantial ore losses or, alternatively, substantial gangue admixture, depending on which of the two extremes is preferred.
(3) Cutting and filling hollow spaces obtained by extraction, by using valueless material such as gangue, sand, waste from mineral processing plants (dressing plants) etc. This filling is planned and artificial, and it presupposes access to suitable fill material. Thus, it is expensive and at times even very expensive. The method, consequently, presupposes that the mineral to be mined is relatively valuable, which implies that mining losses must be small and/or that the subsequent processing of the product in mineral processing (dressing) plants is relatively expensive so as to permit utilization of the other great advantage of the cut- and-fill method, viz. low gangue admixture. The method, thus, is selective. It also may have a third advantage, which more recently has become important, i.e. its environment protective effect, because certain grain size fractions of the waste from subsequent mineral processing (dressing) plants can be recycled to the mine and be used as fill. The fill is supplied to the mine with the object of stabilizing the hollow spaces and preventing them from collapsing.
The present invention relates to a method of underground mineral mining or preparing rock cavities, at which method resulting cavities or hollow spaces entirely or partially are filled with stabilizing ice, either temporarily or, when desired, permanently, but normally temporarily as will become apparent from the following.
The method substantially can be correlated to method (3) where the conventional filling masses are replaced by ice, and it can, like method 3 in its general form be applied to method (1), i.e. mined parts in a mineral deposit can be filled with ice, whereafter the pillars can be recovered. The method according to the invention also can be applied to preparing and mining rock cavities for the storage of solid, gaseous or liquid material.
For facilitating its understanding and describing it more simply, the invention in the following is compared explicitly or implicitly with so-called cut- and-fill mining of mineral deposits, in other words with the mining method, at which the mined mineral deposit or extractable parts of a mineral deposit have been replaced for stabilizing purposes by artificially supplied fill, more precisely by fine-grained material, which in aqueous suspension had been supplied to the mined deposit or parts thereof, so-called filling with hydraulic fill. For elucidating the invention still more and describing it in greater detail, in the following the invention is compared with the cut-and-fill method by using hydraulic fill which, besides, was stabilized by the addition of a binder in order to render possible both so-called upward and downward filling. The binder most usually being cement, the invention primarily is compared with cut- and-fill mining by using cement-stabilized hydraulic fill. The invention, thus, is compared with, and its applicability is described in comparison with the mining of mineral deposits, but, as mentioned, the utilization of the invention is not restricted only to the mining of mineral deposits. The mining and preparation of rock cavities in general also can be facilitated by applying the invention.
As mentioned above, at the cut-and-fill method fill masses, according to the invention ice, are supplied to mined rock cavities for stabilizing the same. The invention can be applied to both upward and downward mining direction with the cut-and-fill method of mineral deposits.
For obtaining a definite size of the mining spaces, the strength of the rock and the minerals as well as of the ice must be taken into consideration.
When mining a mineral deposit by the cut-and-fill method and in upward direction, the strength of the mineral-bearing rock is the limiting factor with respect to the width (span) in a so-called mining space (layer). The height of the layer is determined by the width of the mineralized zone and by the strength of the mineralized rock and/or the strength of the so-called wall rock. When using downward cut-and-fill mining with cement-stabilized hydraulic fill, also the strength of the hydraulic fill is a layer span limiting factor.
Due to the fact that, as will be shown later, one of the advantages of the invention is the applicability of downward mining direction, and as at normal cut-and-fill mining with cement-stabilized hydraulic fill and the use of reasonable cement quantities (mean mixture ratio cement:fill 1:6), the approximate span of the cavities is 6 m, provided its permission by the width of the mineralized zone, the cavity span in the following is assumed to be 6 m. A space height of 4 m has been assumed usual and normal.
It was mentioned above that the caved mining spaces temporarily or permanently were filled with ice. Although the invention per se does not intend to permit the ice to melt in a space while work is going on in spaces immediately below or, with other words, the term "temporarily" covers a relatively short time interval, calculations carried out have proved that an ice beam (ice layer) of 6 m width and 3 m height resists with ample margin the spontaneous load of immediately overlying and collapsing rock.
This situation is a theoretical one, because rock collapse and downward mining direction will not be permitted while work is going on, but is prevented by retaining the ice masses.
The theoretical case described, however, shows that the invention meets the safety requirements. Besides, that the work safety beneath an artificial ice roof is greater than beneath a natural rock roof, should be obvious, as it, too, is obvious that the assumed space size parameters of 6 m width and 4 m height can be reduced according to the configuration of local mineral deposits and special wishes.
When, however, the mineral deposit configuration is of such a nature that the space width should substantially exceed about 6 m, mining in multiples of adjacent spaces must be applied.
Previously it has been proposed to utilize ice for the above purpose, but to our knowledge the proposals never have been realized in production operation.
One proposed method uses ice, which was permitted at an open pit to flow (creep) into the hollow spaces created by mining. The method, thus, presupposes an opening of substantial area and vertical distance between the pit and the earth surface, and further requires access to natural ice on the earth surface and/or at least a relatively cold climate for being able to make the required fill ice without great losses. For rendering it possible for the ice to creep into the pit with sufficient speed, a sufficiently high overlay pressure (ice thickness) is required, and the temperatures must not be too low, which is somewhat contrary at least to the requirement of having a relatively cold climate for being able, if necessary, to make ice by watering the open pit.
The method, besides, has the disadvantage that its utilization requires relatively large mineral deposits, and that it permits mining only in downward direction and always in direct connection to the overlying ice. The method, thus, lacks any possibility of selective mining, i.e. mining in any optionally selected place underground.
The production pace in the mine, besides, depends directly on the creep speed of the ice and, thus, cannot be controlled unconditionally.
The method suitably can be applied at the mining of large mineral deposits according to the sub-level caving method.
Another known method is related somewhat more closely to the present invention and permits a certain selectivity, due to the fact that relatively high vertical cavities are permitted being filled with snow, which by its own weight is compacted to ice at least in the lower layers. The snow is produced by blowing preferably cold atmospheric air through a water curtain. The method of producing the necessary snow at high temperature is only indicated by the statement, that refrigerating units can be used, without giving any detailed instruction.
Ice losses by natural melting are balanced by adding snow or are reduced by heat-insulating the ice body against the surrounding rock. Application of artificial cooling through cooling coils and channels is indicated.
The main object of the present invention is achieved in that the created cavity, in a first step, for a certain period is prepared for ice filling by partially removing the geothermic heat content in the cavity walls, so that the walls have a temperature below 0.degree. C., and in a second step water, possibly together with material increasing the ice strength, for example fine-grained or fibrous material, is supplied in layers and intermittently to the cavity, while the added water together with possibly added material increasing the ice strength is being cooled and frozen, and in a third step the frozen ice body is maintained by removing the geothermic energy constantly flowing in during a period deemed necessary for achieving the object, and that said cooling and freezing in all three steps preferably is carried out with artificially cooled air, but that the first and the third steps can be abolished when the climatic conditions are such, that the rock about the cavity is frozen sufficiently below the freezing point, so that ice of high strength is obtained in a natural way, and that at suitable climatic conditions also the cooling in an artificial way in the second step can be imagined to be abolished.
It seems reasonable to assume that under such conditions also the atmospheric air has such a low temperature as to permit its lower heat content being utilized directly, without help by artificial cooling. The cooling air, for understandable reasons, in all three steps must be given a temperature below 0.degree. C., suitably below -5.degree. C., preferably below -10.degree. C. and most preferably below -15.degree. C., so that the ice has sufficient strength and low creep. These physical temperatures of the ice, namely, increase or decrease substantially at temperatures below -10.degree. C.
When the temperature of the atmospheric air is not in agreement with the aforesaid, artificial cooling of the air or a combination of artificial cooling and natural cold air is applied, possibly depending on the season.
The principle, however, still is that the cooling air is given the necessary low temperature by artificial cooling (cooling-coil batteries), and that low geothermic rock temperature and low atmospheric air temperature are positive factors for the overall economy of the invention, but are not fundamental requirements.
Calculations carried out prove that the total costs of the method are of the magnitude half the total costs of cut-and-fill mining with cement-stabilized hydraulic fill, even when the mean temperature of the atmospheric air is about +30.degree. C. (and the geothermic rock temperature at the same time is about +30.degree. C.). The application range of the invention is thereby practically almost independent of the geographic location of the place of application.
The cooling air generally and in principle must flow in a closed system, entirely separated from the normal mine ventilation system, in order to create acceptable climatologic working conditions for the personnel and, due to the cooling air being in a closed system, to provide the prerequisites to more simply check and maintain cooling air volumes, cooling air temperature, etc.
In order to ensure a reasonable freezing time for the second step, the walls of the cavity in the first step should be frozen by flowing air to a layer of at least some decimeters thickness to a temperature below 0.degree. C. Thereby a cold barrier is established against the geothermic heat flowing in and against the heat from the freezing water added in step two.
The conditions within the cold barrier, viz. its depth and the configuration of the temperature gradient over the frozen barrier depth (thickness), can be varied depending, among others, on the temperature levels of the geothermic heat and of the water as well as on the fixed lengths of the subsequent periods of ice-freezing and of cooling for maintaining the ice. This preparatory freezing preferably should be carried out so that, when the temperature is measured 0.5 m inside of the defining surface between cavity and rock into the rock, the temperature there should be -3.degree. C. at maximum, preferably lower.
The second step then can be commenced. The water added in the second step suitably should have a temperature immediately above 0.degree. C., preferably about +1.degree. C. to +2.degree. C., and the water preferably is spread intermittently and in layers. The flowing cooling air in the second step preferably is added also intermittently, substantially in pace with the spread of water, so that the cooling air entirely or partially is stopped during the water spread periods. When no spread takes place, the cooling air may flow at full speed and freeze the added water layer.
The cooling air shall cool as efficiently as possible. The cooling air, therefore, suitably by means known per se, for example by fans, guide bars or dampers, and advantageously assisted by process computers, should be given such a speed in the closed system that the cooling energy required for the different steps is supplied to the contact surfaces exposed to cooling, so that the surfaces within a reasonable time assume the desired temperature during the period in question of the method step. At given parameters such as the geothermic temperature of the rock, the desired configuration of the artificially created cold barrier against the geothermic heat, the temperature of the atmospheric air, the electric energy price etc., the question of the necessary cooling air quantity, and therewith the speed of the cooling air, is a problem of optimization.
The cooling air flows to the mining spaces through conduits, usually drifts or raises. The lastmentioned ones usually have a cross-sectional area of about 10 m.sup.2, which also has been assumed as calculation basis for the invention. Due to aerodynamic conditions, the air speed in these drifts and raises suitably should be limited to about 10 m/s. When the air enters the mining space, which has a cross-sectional area of about 24 m.sup.2, the air speed drops to about 4 m/s. This speed cannot be regarded satisfactory and, therefore, turbulence is created in the air stream by auxiliary fans, guide bars and the like, so that the air along the contact surfaces exposed to cooling assumes a speed of about 10 m/s, at which speed the heat transfer is improved (i.e. cooling energy is supplied more efficiently). Thereby, the cooling can take place in a shorter time than when the speed was only 4 m/s. In principle, however, any cooling air speed and any cooling air volume, within reasonable limits, can be chosen.
During the ice freezing, other requirements on cold air stream adjustment arise. When the water spreading is chosen to be carried out intermittently, first of all it must be possible to stop, or to very substantially reduce the air flow while the water is being spread. Furthermore, even during the intervals, i.e. when no water is being spread, adjustment must take place in order to obtain the correct air speed and air volume later on when the ice layer grows in thickness, i.e. the cross-sectional area decreases. The adjusting can be controlled by scanning means known per se, which emit sinals, for example to a process computer, which according to a predetermined programming emits signals so, that the desired adjustments are carried out. All this takes place in agreement with known process control technology.
The ice freezing or ice production step at downwardly directed mining is carried out so that a gap (cooling gap) remains between the upper defining surface of the frozen ice beam and overlying rock or ice beam. The gap has a height of about 1 m and, thus, a cross-sectional area of about 6 m.sup.2. The gap is intended for the third step, the so-called maintenance-cooling, and thus is a part of the aforesaid closed system, through which the cooling air is to circulate during the period deemed necessary for maintaining the ice for stabilization purposes.
At upward mining direction, where the ice is the sole in a mining space, the possibility of making this cooling gap in an equally simple way does not exist.
When under upward mining direction conditions a cooling gap is deemed necessary, it can be provided by placing a simple thin-walled metal sheet, plastic or cardboard, tubes or boxes on the sole of a mining space before the ice production is commenced.
Another possibility is to drill vertical bores of a suitable diameter through the ice and thereby bring about vertical cooling slits for maintenance cooling.
It may happen that the rock proper holds a temperature so low, that maintenance cooling is not required. In this case, as already mentioned, the third step is abolished.
The present method in principle is based on an artificial cooling of the cooling air to preferably below -10.degree. C., most preferably -15.degree. C. and at times to, for example, -25.degree. C. For overall economic reasons it may be desirable occasionally to fall below -15.degree. C. to, for example, -25.degree. C. in order to increase the mining capacity for a given mine or mine section. When due to high geothermic rock heat preparatory cooling (pre-cooling) of a space is applied, and when possibly also later on the produced ice is subjected to maintenance cooling, for the same reasons under conditions of prevailing low mean temperatures of the atmospheric air, the heat removed from the rock, of course, can be supplied to the normal mine ventilation air by help of the cooling air for its possible heating through the heat exchanger in the closed cooling air system.
At the ice production in the second step, material increasing the strength of the ice (reinforcing additives) possibly may be admixed to the water.
It was found by experiments that, for example, the bending strength of the ice can be increased by up to 200% by the admixture of about 10 percent by weight of fine-grained material. The increase in strength increases significantly at decreasing grain size, at least within the range 0.1-0.05 mm. It was found suitable to use fine-grained material originating, for example, from dressing plants at mines. Extremely fine-grained material fractions in the waste from these plants when being deposed, for example, in waste pools gives rise to troubles unless special measures such as pH adjustment and availability of a large waste pool surface are taken. In this connection problems with metal ions have been observed.
Environmental problems arising from the aforesaid negative factors, viz. fine-grained waste material and metal ions, can be solved partially, in certain cases probably completely, by using waste water from the dressing plants as water for the ice fills. Super-fine waste grain fractions, which probably have no strength-increasing effect for the ice, are baked into the ice, rendered harmless and remain in the mine even after the ice was permitted to melt.
It is known previously and can be applied also in this case, that fibre reinforcement can increase the tensile strength of the ice by more than 100%, when about 10 percent by volume of organic fiber, for example wood fiber, is admixed.